This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 illustrates a prior art emitter switched bipolar transistor. Such circuit arrangements may have higher switching speeds comparable with MOSFETS, have lower saturation voltages for high voltage applications, and may be lower in cost. They may be used in power chargers or adapters. The low voltage MOSFET Q2 is controlled by the high frequency PWM control signal and turns on or off current flowing through the emitter of the BJT Q1. The BJT is driven by a fixed base drive appropriate for on-time operation based upon gain characteristics of the device.
Switching BJTs may take a long time for the collector voltage to fall after applying a base current, which may lead to higher turn on losses as the collector current starts to build while the BJT is still in linear mode. A large base current overdrive may be used to turn on the BJT and saturate it rapidly to enhance turn on performance.
FIG. 2 illustrates a prior art emitter switched bipolar transistor having a capacitor C1 coupled to the base of the BJT Q1. When a BJT is driven at a base current higher than necessary to keep it in saturation, excess base drive energy is stored in the base region, which may cause slower turn off and energy loss due to over drive. In operation, when the control MOSFET Q2 turns off, emitter current of the BJT Q1 is turned off rapidly and the collector current diverts out of the base till all storage charge is removed. The capacitor C1 may be used to recover storage charge and use it in a regenerative manner. The collector current coming out of the base in a reverse direction charges capacitor C1. Therefore, after turn off, the voltage on capacitor C1 may be higher than the bias voltage supply V1. During the next turn on instance, a base current pulse may be delivered by energy stored in capacitor C1 through resistor R1. A zener diode Z1 may be used to limit the voltage on capacitor C1.
FIG. 3 illustrates a prior art emitter switched bipolar transistor having a current transformer TX1 to provide a proportional base drive current. The storage energy returned by diverting collector current out of the base is used to charge capacitor C1 to a desired voltage level. The voltage level may vary depending upon the stored energy in the base region, parametric variations and the value of the capacitor C1. The voltage may be limited by the zener diode ZD1, which may dissipate the remaining energy when its break down voltage is exceeded.
Each of the above prior art circuits require a capacitor storage element and thus cannot be implemented in an integrated circuit.
FIG. 4 illustrates a prior art power converter having an emitter switched bipolar transistor. The power converter is a typical buck converter with a control switch placed on the low side of the DC input. Bias supply voltage V1 is a high voltage input DC source. The emitter switched bipolar transistor comprises switches Q1 and Q2. Diode D1 is a freewheeling diode. Inductor L1 is a buck output inductor. Capacitor C1 is a filter capacitor. Various parasitic capacitances are also illustrated: C2 is the control MOSFET's (Q2's) drain to source capacitance; C3 is the BJT's (Q1's) collector emitter capacitance; C4 is the inter-winding shunt capacitance of the inductor L1; C5 is the body capacitance of the freewheeling diode; C6 represents any parasitic capacitance from collector to return.